Non-linear resistance material



KILOVOLTS Dec. 13, 1966 .1. J. PlTHA 3,291,759

NON-LINEAR RESISTANCE MATERIAL Filed Dec. 15, 1963 Alzo3y @l (MULUTE) 2M9o 2A12035sfo2 (CORDIERITE) 100 QMPERES fmfzff;

United States `Patent O 3,291,759 NON -LINEAR RESISTANCE MATERIAL John J. Pitha, Lenox, Mass., assignor to General Electric Company, a corporation of New York Filed Dec. 13, 1963, Ser. No. 330,387 6 Claims. (Cl. 2512-516) This invention relates to resistance material and more particularly to non-linear resistance material for use as valve elements in overvoltage protective devices, such as lightning arresters, and to a novel method of making such material.

As is well known, overvoltage protective devices, such as lightning arresters, generally utilize a gap structure arranged in series with a type of non-linear resistance material, usually termed a valve element. When subjected to overvoltage, such as a lightning or a'switching surge, the gap structure arcs over, forming a low -resistance path to ground through the non-linear resistance material. The non-linear resistance material provides a low resistance path to high voltages and a high resistance path to low voltages. Thus, on overvoltages when the gapstructure arcs over, the voltage is readily conducted to ground through the low resistance path provided by the non-linear resistance material. When the overvolta-ge surge has been discharged the non-linear resistance material provides a high resistance path to thel power follow current, limiting such current to enable the gap structure to interrupt the current and return the arrester to its open circuit condition.

As is well understood by those skilled in the art, the lower the resistance of the non-linear resistance material to surge voltages the lower will be the potential developed across the resistancematerial. The potential developed across the resistance material is effectively the potential to which any associate electrical apparatus is subjected on lightning and switching surges. Obviously, it is desirable to obtain the lowest possible resistance to such surges to limit this potential, to prevent damage to any connected electrical equipment. Of course, since the resistance material must pass the entire surge, it is necessary that the material also be sufficiently strong electrically to pass these surges without rupturing or arcing over and without change in its electrical characteristics.

It has recently been discovered that a novel, non-linear resistance material can be made having improved low resistance characteristics by utilizing a cordierite bond to bond the material. The use of a cordierite bond also provides a much lower thermal coelicient of expansion thereby improving the electrical strength of the resistance material.

It is, therefore, one object of this invention to provide a new and novel non-linear resistance material.

A further object of this invention is to provide a new non-linear resistance material for use in overvoltage protective devices having low resistance to overvoltages.

A still further object of this invention is to provide a new non-linear resistance material having a novel bond to provide increased electrical strength.

A still further object of this invention is to provide a new non-linear resistance material having novel bonding characteristics which provides very low resistance characteristics with increased electrical strength.

Briefly, this invention in one form comprises a nonlinear resistance material consisting of approximately 65 parts by weight of a silicon carbide material, 14 to 21 parts by weight of talc and 21 to 14 parts by weight of porcelain mixture; that is, the non-linear resistance material consists of approximately 65% by weight of silicon `carbide material and 35% by weight of binder material.

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The invention which is desired to be protected will be particularly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its various objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood by reference to the following detailed `dsecription of preferred embodiments thereof, particularly when considered in connection with the accompanying drawing, in which:

FIGURE 1 is a portion of a ternary diagram for MgO*Al2O3-Si02 showing the mullite and cordierite areas of such diagram; and

FIGURE 2 is a graph showing the comparative voltamperes characteristic of the non-linear resistance material of this invention with a non-linear resistance material of the prior art.

As was earlier noted, it has 4been discovered that a novel non-linear resistance material can be obtained having improved electrical characteristics by use of a cordierite bond. Tests which have been performed on nonlinear resistance material using essentially a cordierite bond show that the non-linear resistance material has lower resistivity than similar material of the prior art. As is well known, cordierite is generally indicated as 2MgO.2Al2O3.5SiO2. The cordierite material is designated by a particular area of the ternary diagram for A1203, SiO2 and Mg() and this particular area is shown in the ternary diagram of FIG. 1, marked Cordierrte. The prior art non-linear resistance material, suc-h as for example that disclosed in the Evans et al. Patent No. 2,529,144, in general rely on mullite as the bonding agent. Mullite is indicated as 3Al2O3.2SiO2 and is found in the area of the ternary diagram which is marked Mullite in FIG. 1 of the drawing. In general, it may llne said that it has recently been discovered that, for a non-linear resistance material, a cordierite bond is superior to a mullite bond both in low resistance characteristics and in thermal expansion characteristics.

FIGURE 2 shows curves of the volt-ampere characteristcs of a ycordierite bonded non-linear resistance mate- `rial made according to this invention and a non-linear resistance material using the mullite bond such as shown in the Evans et al. Patent No. 2,529,144. Curve A of FIG. 2 is that of a non-linear resistance disk made in accordance with Ithe disclosure of the Evans et al. patent. The curve is an indication of the average volt-ampere characteristics of a 6 diameter disk 1.1 high which is rated at 3 kv. Curve B is the average volt-ampere characteristics curve for a non-linear resistance disk made according to this invention. The disk used in Curve B was 3" in diameter, 1.1l -high and was also rated at 3 kv. It will be readily apparent from FIG. 2 that Curve B exhibits substantially improved resistivity over the disk of Curve A. This is even more significant when it is realized that Curve A is for a 6" diameter disk, while Curve B is for a 3" diameter disk. As will be readily apparent to those skilled in the art, the 6 diameter disk would be expected to exhibit a lower resistance than the 3 diameter disk. Thus the curves of FIG. 2 provide a graphic indication of the superior resistance characteristics of the non-linear resistance material of this invention.

In preparing the non-linear resistance material of this invention, approximately 65 parts by weight of silicon carbide are mixed with from 14 to 21 parts by weight of talc and from 21 to 14 parts by weight of a porcelain mixture. As is well understood by those skilled in the art, a porcelain mixture, in general, comprises a flint material, clay materials and a soda spar material land nds great use in the porcelain art. Any particular type of porcelain mixture may be utilized with this invention.

After the above materials have been thoroughly mixed, they are pressed into disks of the desired size for the particular overvoltage device. The disks are then tired at a temperature of from 1240 to l260 C. in a hydrogen atmosphere. In a preferred form of the invention the inlet hydrogen atmosphere is at a -80 dewpoint. The resultant disks exhibit exceptionally low resistance characteristics such as, for example, those characteristics disclosed in Curve Bof FIG. 2.

It has been found that it is desirable that the total amount of silicon carbide material be approximately 65% by weight of the total mixture. When the percentage of silicon carbide -exceeds approximately 65 of the total mixture, desirable electrical characteristics are lost. The same is true when the amount of silicon carbide material in the total mixture is much less than approximately 65% of the total mixture. When the amount of silicon carbide exceeds approximately 65% there is a substantial loss in the long duration electrical strength of the disks made from the material, while when less than approximately 65% of silicon carbide is used the non-linearity of resistivity of the disks falls off rather rapidly. Therefore, as set forth above, it is generally desirable that the amount of silicon carbide mixture in the non-linear resistance material be approximately 65% of the total mixture.

Below are given specific examples of the non-linear resistance material which may be made according to this invention:

Example I Ingredients: Parts by weight Silicon carbide (60 to 180 grit) 65 Talc 14 Flint 3 Clay Soda or Potash Spar 8 After thorough mixing of the above ingredients, -with suitable temporary binders, such as water, the mixture was pressed into disks of the derised size and shape. The disks were then fired at approximately 1250" C. in a hydrogen atmosphere.

Example I1 Ingredients: Parts by weight Silicon carbide 65 Florida kaolin 17.5 Talc 17.5

The above ingredients were thoroughly mixed with suitable temporary binders, such as water, and pressed into disks of the desired size yand shape. The disks were then red at about 1240 C. in a hydrogen atmosphere.

Disks made according to this invention, such as in the manner of either of the above examples, find utility in various types of overvoltage protective devices since they provide a very low resistance path for surge voltages. While there has been shown and described the present preferred embodiment of the n-on-linear resistance material yof this invention, and the preferred method of making such material, it will be understood that various modifications may be mad@ Without departing from the spirit and scope of the invention particularly as set forth in the appended claims.

What is claimed as new and which it is desired to secure by Letters Patent of the United States is:

1. A method of making a non-linear resistance material for use in overvoltage protective -devices compri-sing the steps of mixing approximately parts by weight of silicon carbide with 14 to 21 parts by weight of talc and 2l to 14 parts by weight of a porcelain mixture, forming said mixture into shaped articles and firing the articles at a temperature from 1240 t-o l260 C. in a hydrogen atmosphere.

2. A method of making a non-linear resistance material for use in overvoltage protective devices comprising the steps of mixing approximately 65 parts by weight of silicon lcarbide with 17.5 parts by weight yof Florida Kaolin and 17.5 parts by weight of talc, forming said mixture into shaped articles and ring said articles at a temperature from 1240 to 1260 C. in a hydrogen atmosphere. 3. The method of making non-linear resistance material for use in overvoltage protective devices comprising providing a mixture of silicon carbide particles and a cordierite-forming binder material therefore, said mixture consisting essentially of approximately 65 parts by weight of silicon carbide, 14-21 parts by Weight of talc, 21-14 parts by weight of a porcelain mixture, compacting said mixture, and tiring said mixture at a temperature of about 1240 C. to about 1260 C. in a hydrogen atmosphere.

4. A non-linear resistance material having excellent low resistance characteristics and a highv capacity for withstanding repeated current surges for use in overvoltage protective devices, said material comprising a mixture of approximately 65 parts by weight of silicon carbide particles and approximately 35 parts by weight of cordierite binder therefor, said material being made Aby the process defined in claim 3.

References Cited by the Examiner UNITED STATES PATENTS 1,927,894 10/1933 Krause 252-516 k2,529,144 11/1950 Evans et al 252-516 2,796,505 6/ 1957 Bocciarelli 252-516 2,962,452 ll/ 1960 Counts et al 252-520 y3,162,831 12/1964 Heath 252-516 SAMUEL H. BLECH, Primary Examiner. J'ULrUs GREENWALD, Examiner.

J. D. WELSH, Assistant Examiner1 

3. THE METHOD OF MAKING NON-LINEAR RESISTANCE MATERIAL FOR USE IN OVERVOLTAGE PROTECTIVE DEVICES COMPRISING PROVIDING A MIXTURE OF SILICON CARBIDE PARTICLES AND A CORDIERITE-FORMING BINDER MATERIAL THEREFORE, SAID MIXTURE CONSISTING ESSENTIALLY OF APPROXIMATELY 64 PARTS BY WEIGHT OF SILICON CARBIDE, 15-21 PARTS BY WEIGHT OF TALC, 21-14 PARTS BY WEIGHT OF A PORCELAIN MIXTURE, COMPACTING SAID MIXTURE AND FIRING SAID MIXTURE AT A TEMPERATURE OF ABOUT 1240*C. TO ABOUT 1260*C. IN A HYDROGEN ATMOSPHERE. 